Not sure you are allowed to carry passengers during acceptance tests. Nexus will not have accepted them as fit for service until a certain amount of test hours have been completed. They have to check the interface with the signalling system, communication systems and how the new and old fleet operate together. They have to test to make sure they fit the loading guage. I believe there is one metro that isn't in gauge! They discovered problems on Merseyrail when the new and old fleet operated to together. These still belong to Stadler until signed off. Would you fly in an untested aeroplane?
Thought not!
You can't carry passengers until testing is complete, and both the type and the individual unit are accepted into service. Type acceptance involves getting approval from the ORR.
Individual unit acceptance is based on the contractual performance benchmarks agreed between Nexus and Stadler, above and beyond what is required to comply with the acceptance criteria the ORR have set. Even if this were not the case, the unit doesn't actually belong to Nexus until they accept it and formally take delivery.
I very much doubt that one of the units is not within gauge. This stuff is all checked in the factory as each unit is produced and they are manufactured on jigs, so for a single example of a class to have such an issue would be highly unusual. Loading gauge testing is now more or less complete from what I gather, with the units now having reached every extent of the network.